1. Field of the Invention
The present invention is in the field of semiconductor materials, and more particularly thermoelectric materials and devices made with highly [111]-oriented twinned group IV semiconductor alloys on the base plane of trigonal substrates.
2. Description of the Related Art
Solid-state thermoelectric devices convert temperature differences into electricity, or develop a temperature difference when a voltage is applied. Thermoelectric effects in semiconductor junction devices include (among others) the Seebeck Effect. The Seebeck Effect is due to a combination of charge carrier diffusion and phonon drag.
Charge carrier diffusion, in semiconductors made from n- and p-doped materials, results when electrons and holes diffuse in response to a temperature gradient. The diffusing charges are scattered by impurities, imperfections and lattice vibrations (phonons). Since the scattering is energy dependent, a charge density differential will develop in response to a temperature gradient. The magnitude of the effect tends to increase with the electrical conductivity of the material, and to decrease with its thermal conductivity. However, there are interdependencies of these characteristics, which have added to the difficulty of developing good thermoelectric materials.
While good semiconductor device materials require a single crystalline phase without defects, many good thermoelectric materials have electrically connected poly-type crystalline structures that scatter phonons, thus reducing thermal conductivity. For example, thermoelectric skutterudite material has three pnictogen square planes that can orient randomly. See M. Formari, D. J. Singh, I. I. Mazin, and J. L. Feldman, in MRS Symposium Proceedings 626, Thermoelectric Materials 2000—The Next Generation Materials for Small-Scale Refrigeration and Power Generation Application, 2000, p. Z6.3.1. One material that has particularly been employed for thermoelectric purposes is Skutterudite CoSb, which has many poly-type structures to scatter phonons, yet the structures are electrically connected.
As another example, U.S. Pat. No. 6,207,886 teaches the use of a Co—Sb-based filled-skutterudite sintered material having a lower thermal conductivity and thereby having a higher figure of merit. This reference teaches a formulation involving fine crystals, in which the areas at the boundaries of the fine crystal grains are increased, phonon scattering is enhanced, the thermal conductivity is decreased, and the figure of merit is increased.
Thermoelectric devices have been made from other materials, such as silicon-germanium (SiGe), but these have conventionally exhibited lower figures of merit. While Skutterudite CoSb has a good figure of merit, it can perform only at intermediate operating temperatures. Because electric output is proportional to operating temperature, SiGe thermoelectric materials have been preferred for many applications because of their ability to operate at higher temperatures. Similar performance has been seen with other group IV variants, including various alloys of silicon, germanium, carbon and/or tin. However, previously known SiGe and other group IV thermoelectric materials have a relatively low figure of merit due to their high thermal conductivity.